Generally, a fluorescent lamp is so constructed that electrodes are disposed at both ends of a linear glass tube and luminescence is caused by exciting gas contained in the glass tube due to electric discharge between the electrodes. The luminescence efficiency increases as the distance between the electrodes, namely, the length of electric discharge path is prolonged. Accordingly, if luminescence efficiency is considered to be important, use of a longer glass tube is advantagaeous. However, discharge lamps are used as light sources at a variety of positions and it may be desirable for users to have one having a compact structure which is convenient to handle it. On account of this, various proposals have been made as to a discharge lamp with a curved glass tube in order that luminescence efficiency is maintained at a high level to some extent while the size of the lamp is reduced by making its structure compact.
FIG. 1 shows a U-shaped fluorescent lamp as a typical example of the discharge lamp of this kind, in which glass stems 3, each holding an electrode 2, are sealingly provided at both ends in a U-shaped glass tube 1 having an inner wall on which a fluorescent layer 11 are formed.
The fluorescent lamp having the construction described above is utilized in fields requiring a compact light source since such lamp imparts substantially the same luminescence characteristic as an ordinary fluorescent lamp using a linear glass tube although the length of the lamp is one-half.
However, the discharge lamp having the construction as above-mentioned has the disadvantage that manufacturing steps are complicated in comparison with the conventional discharge lamp using a linear glass tube. Particularly, it is difficult to attach a glass stem to the end portion of the glass tube 1 to seal it. Namely, a general method of seal-bonding a glass stem 3 to the end of the glass tube 1 is such that the glass stem 3 is brought to contact with the end of the glass tube which is previously bent into a U-shape; heat is applied to the periphery of the contacting part by a gas burner (not shown) to soften that portion thereby causing melt-bonding of the glass stem to the glass tube.
In the discharge lamp having the shape as shown in FIG. 1, however, since both parts to be sealed are placed contiguously each other, it is difficult to apply heat from the burner to those parts to thereby possibly cause defect such as cracking during sealing operations. Further, the conventional discharge lamp is not always sufficient in view of making the structure compact because a linear glass tube is bent only one time.
FIG. 2 shows a fluorescent lamp aiming at its compact structure disclosed in Japanese Unexamined Publication No. 83147/1980 in which the fluorescent lamp has a base plate 23 whose one surface is attached with a lamp base 9 integrally and whose other surface is firmly secured to a glass tube 1 as a luminescence tube. The glass tube 1 is formed into a saddle shape in general by bending a linear glass tube at its intermediate portion into a U-shape and further bending the U-shaped glass tube by 180.degree. in the direction perpendicular to the axis of the bending. Reference numerals 3 designate stems attached to and sealed in both ends of the glass tube 1 and numerals 2 designate electrodes extending from the stems 3.
In the discharge lamp having the construction as above-mentioned, bending operations of the glass tube are more complicated because a single linear glass tube is bent into a U-shaped glass tube and it is further bent into double U-shapes. Furthermore, in the second bending operations, two portions in the glass tube are simultaneously bent and accordingly, if a large curvature is taken, there may result in defect in the glass tube formation. Therefore, there has been restriction in making structure of discharge lamps compact.